University of Arkansas - AgriScience Project

AGRISCIENCE EXERCISE

PHYSICAL SCIENCES IN AGRICULTURE

Key Concept: Energy & Power Systems

Sub-Concept: Power Transmission

Agricultural Application: The agricultural industry relies on the efficient transmission of power in mechanical systems.

Exercise: Measuring Work and Power

Applied Principle(s): Work and Power in Mechanical Systems

Goals:

  1. Define work and power.
  2. Identify units of measurement of work and power.
  3. Calculate the amount of work done and the amount of power developed in a given situation.


Materials:



References: Olivo, C.T. and Olivo, T.P. (1984). Fundamentals of Applied Physics, Third Edition. Albany, NY: Delmar Publishers, Inc.

Also refer to the attached information sheet or any high school physics textbook

Procedures for Conducting the Activity:

1. Divide the class into pairs of students, and provide each with a data sheet and the necessary materials for this exercise.

2. Instruct the students to complete the activity as directed on their data sheets. You may wish to monitor their progress as they work; however, it is suggested that the students be left to follow the instructions and complete the activity on their own.

3. Once all groups have completed the exercise, discuss the answers to the discussion questions as a class. Be sure to make note of the practical agricultural applications of the principles demonstrated.







AGRISCIENCE EXERCISE

- Measuring Work & Power -

Student Data/Instruction Sheet

1. Obtain the following equipment and supplies:



2. Use the spring scale to determine the amount of force (lbs.) required to lift the weight. Record this measurement below:

Force = __________ lbs.

3. Measure the distance from the floor to the top of the table at your work area. Record this measurement below:

Distance = __________ft.

4. Lift the weight from the floor to the top of the table. Calculate the amount of work done, using the formula below. (Show your calculations and draw a box around your answer.)

Work (ft.-lbs.) = Force x Distance







5. Using the chalk and tape measure, mark off a distance on the floor equal to the distance recorded in Step 3.

6. Use the spring scale to pull the weight through the distance marked off in the previous step (Step 5). Determine the average force required to pull the weight. Record below:

Force = __________lbs.



7. Calculate the amount of work done in pulling the weight through the measured distance. (Show the formula and your calculations. Express your answer in correct unit. Draw a box around your answer.)















8. Place the weight on the floor again. Since the force and the distance recorded in Steps 2 and 3 have not changed, simply record these numbers in the blanks below:

Distance = __________ft.

Force = __________lbs.



9. Determine the time required to lift the weight from the floor to the table. Record the time in the blank below:

Time = __________minutes (or decimal parts of a minute)



10. Use the formula below to calculate the amount of power produced in lifting the weight. (Show your calculations and draw a box around your answer.)

Power (ft.-lbs. per minute) = Force X Distance

Time (min.)









11. Pull the weight through the measured distance marked on the floor, noting the time required. Calculate the power required. (Show the formula and your calculations. Express your answer in correct unit. Draw a box around your answer.)











12. Answer the following discussion questions. Be prepared to discuss your answers with your classmates.

a. Did it require more work to lift the weight through a distance or pull the weight through the same distance? __________ Why was this true?







b. What is the unit of measurement for work? _____ for power?_____





c. How much work would be accomplished if you applied 1000 lbs. of force against an immovable object? __________ How can you justify your answer mathematically?













d. What factor most limited the power you were capable of producing when you lifted the weight from the floor to the table?

TEACHER BACKGROUND SHEET

- Measuring Work & Power -

In the United States, the horsepower unit is the commonly used measure of the power output of internal combustion engines and electric motors. By definition, one horsepower is the ability to do 33,000 foot-pounds (ft-lbs) of work in one minute (or 550 ft-lbs of work in one second). The basic horsepower formula is presented below as Equation 1:

Equation 1: Horsepower = Force, lbs. X Distance, ft.

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Equation 1 is appropriate for calculating horsepower when the force moves in a linear (straight-line) fashion. (In fact, Equation 1 is sometimes called the "linear horsepower equation.") However, since internal combustion engines and electric motors produce a rotary (rotating) output force, Equation 2 (below) is used to calculate rotary horsepower.

Equation 2: Horsepower = Torque, lb-ft. X RPM

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Equation 2 is mathematically derived from the linear horsepower equation (Equation 1). This derivation is shown below:

1. HP = F x D

T x 33,000

2. D = 2pr x RPM

3. HP = F x 2pr x RPM

RPM (revolutions per minute)

Note: Time has been eliminated from the denominator because it is now included as in the numerator.

4. F x 2pr x RPM / 2p Note: 2p = 2 x 3.1416 (or 6.2832)

33,000 / 2p

5. F x r x RPM

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6. To x RPM Note: Force x radius is the same as

5252 force x lever arm length. F x LA = Torque.







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